The advantages of using single crystal semiconductors such as silicon as a mechanical material have long been recognized. For example, it's strength and high intrinsic quality factor make it attractive for MEMS resonant devices. It is regularly available as an integrated circuit substrate and can be processed using methods developed by the IC industry. Recently, the preferred material for forming MEMS resonators is polycrystalline silicon, or simply, polysilicon. This material is advantageous because it is readily used in integrated circuits (often used as transistor gates), provides flexibility in geometry, and ease of use.
MEMS resonators are now being developed for signal filtering and for use as clocks in oscillators. However, for a MEMS resonator, the resonance frequency of the resonator after the manufacturing process is usually different from the desired value due to processing variations. Thus, although one may desire to have a MEMS resonator have a resonance frequency of 1 GHz, during the actual manufacturing process, it is difficult to manufacture a MEMS resonator with exactly a resonance frequency of 1 GHz.
One of the primary parameters that affect the resonance frequency is the dimension of the resonator. While there are post-manufacturing techniques, such as laser trimming, that may be used to adjust the dimensions, and thus the resonance frequency, of the MEMS resonator, this laser trimming is also difficult to accurately control. Therefore, it is costly and/or difficult to precisely manufacture a MEMS resonator having the desired resonance frequency. Another method of adjusting the dimension of a resonator is to use local heating, which will cause expansion of resonator. However, this technique requires a dedicated circuit on the IC to effectuate the local heating.
It has been found that the resonance frequency of a MEMS resonator may be adjusted by applying tensile strain or compressive strain to the resonator. Specifically, the resonance frequency of a resonator will increase when subjected to tensile strain and will decrease when under compressive strain.